Nowadays, it is urgently desired to improvem temperature sensing precision in a temperature sensor. In particular, a high-temperature state in a Li ion battery or CPU is liable to be brought about during operation, and also the operation is liable to become unstable in the high-temperature state. For this reason, there is a high need to sense the temperature within the apparatus accurately and therefore, a temperature sensor having a high precision is demanded.
The temperature sensor, a sensor using Pt, or an NTC thermistor using an oxide of a transition metal element such as Mn, Ni, Co, or Fe is well known. However, the former sensor has the disadvantage of being expensive because the sensor uses Pt which is a noble metal. On the other hand, the latter NTC thermistor has a disadvantage of having a low temperature sensing precision because the resistance change relative to the temperature is small.
As disclosed in the non-patent document 1 or the patent document 1, an element using metal-insulator transition is known as an element that can realize a sharp resistance change.
The non-patent document 1 discloses the electric and magnetic characteristics of a perovskite-structure oxide (RTiO3) of a rare earth element R and titanium where La, Pr, Nd, Sm, and Y are selected as the rare earth element R, or RCaTiO3 in which the Ca, which is an alkaline earth metal element, is dissolved to form a solid solution. As shown in FIG. 5 of this non-patent document 1, the magnetic state of the material system in RTiO3 changes dramatically with a boundary located at a low-temperature region around about 100 K (−173° C.) (which is referred to as metal-insulator transition), thereby exhibiting a behavior of what is known as a strongly correlated electron system. Also, the electric characteristics are shown in FIG. 1 and FIG. 8, where the resistance temperature curve changes around 50 to 150 K.
The patent document 1 discloses a temperature sensor constructed by forming a thin film of vanadium oxide (VO2), which generates metal-insulator transition at about 65° C., on a substrate.
However, the temperature at which the metal-insulator transition in the non-patent document 1 is exhibited is as low as −223° C. to −123° C., and specific magnetism temperature characteristics and resistance temperature characteristics are exhibited only in an extremely low-temperature region. Therefore, the above-described characteristics cannot be used under a temperature (for example, −25° C. to +85° C., which is hereafter referred to as “temperature of actual use”) at which a general consumer may actually use an electronic apparatus.
With respect to the patent document 1, the VO2 thin film exhibits a sharp metal-insulator transition at 65° C., which is within a range of the temperature of actual use. However, as will be understood from the known fact that when a thermal change above and below 65 to 70° C. is given to a VO2 single crystal, the crystal will decay into pieces, VO2 itself has a disadvantage of being extremely brittle. For this reason, there is a problem in that it is difficult to use VO2 in an electronic component. Also, the VO2 thin film in the patent document 1 is fabricated by a thin film forming method, thereby raising a problem of poor productivity.